1. Field of the Invention
The present invention relates to a control system for an internal combustion engine comprising a valve timing changing unit and, particularly, to judgment on a fail of a valve timing changing unit.
2. Description of the Prior Art
FIG. 7 is a structural diagram of a prior art control system for an internal combustion engine. In FIG. 7, a system for changing only the opening or closing timing of a general intake valve as an example.
In FIG. 7, reference numeral 100 denotes a cylinder, 101 a piston which reciprocates in the cylinder 100, 102 an ignition coil, 103 an intake pipe, 104 an exhaust pipe, 105 an injector, 106 an O.sub.2 sensor, 107 a water temperature sensor and 108 a throttle valve.
As shown in FIG. 7, as for the configuration of an internal combustion engine comprising a valve timing changing unit, an intake valve 117 and an exhaust valve 118 for sucking and exhausting air are provided in the top portion of the cylinder 100 in which combustion is carried out, an intake cam shaft 115 for opening or closing the intake valve 117 is arranged above the intake valve 117, and an exhaust cam shaft 119 for opening or closing the exhaust valve 118 is disposed above the exhaust valve 118.
A hydraulic actuator (to be abbreviated as "VVT ACT" hereinafter) which is driven by the lubricating oil of the engine is connected to the end surface of the intake cam shaft 115. This VVT ACT 114 changes the valve opening/closing timing of the intake valve 117 continuously by varying the displacement angle of the intake cam shaft 115 from a timing pulley 120 on an intake side.
An oil control valve (to be abbreviated as "OCV" hereinafter) 121 supplies hydraulic oil to the VVT ACT 114 and controls the amount of hydraulic oil to drive the VVT ACT 114 so as to change the valve timing.
FIG. 7 shows a system for changing only the valve timing on an intake side. The same can be said of a system on an exhaust side.
FIG. 8 is a block diagram showing an example of an engine control unit (to be abbreviated as "ECU" hereinafter) as a prior art control system for an internal combustion engine.
In FIG. 8, reference numeral 1 denotes operation state detection means. This operation state detection means 1 detects the operation state of an engine from the output signals of sensors such as a crank angle position detection sensor 110 for detecting the engine speed of an engine, a throttle opening detection sensor 112, a pressure sensor 113 and a water temperature sensor 107.
Denoted by 2 is actual valve timing detection means. This actual valve timing detection means 2 detects the position of actual valve timing from the output signals of the crank angle position detection sensor 110 and a cam angle position detection sensor 111.
Reference numeral 5 signifies actual advance angle calculating means for calculating the actual advance angle .theta.r of the valve from the detection value .theta.a of the actual valve timing detection means 2 and the storage value .theta.*1 of the first storage means 4.
Denoted by 6 is control means for controlling the valve timing changing unit 7 such that the actual advance angle .theta.r calculated by the actual advance angle calculating means 5 should converge to the target advance angle .theta.b set by the target advance angle setting means 3 and for carrying out feed-back control based on a difference between the actual advance angle .theta.r and the target advance angle .theta.b to output a control signal Y according to the amount of control.
The valve timing changing unit 7 consists of the above VVT ACT 114 for continuously changing the phase of the intake cam shaft 115 with respect to a crank shaft 116 and the above OCV 121 for driving and controlling the VVT ACT 114.
The OCV 121 consists of a spool valve for switching an oil passage to the VVT ACT 114 and a linear solenoid for controlling the position of the spool valve. A current to be supplied to this OCV 121 is controlled by a control signal from the control means 6 such that the amount of hydraulic oil should be adjusted by switching the oil passage to the VVT ACT 114 to drive the VVT ACT 114 so as to change the valve timing.
Reference numeral 8 denotes fail judging means. This means judges a fail (abnormality) of the valve timing changing unit 7 based on the target advance angle .theta.b set by the target advance angle setting means 3 and the actual advance angle .theta.r calculated by the actual advance angle calculating means 5.
A description is subsequently given of the actual valve timing detection operation of the actual valve timing detection means 2 with reference to FIG. 9.
FIG. 9(a), (b), and (c) are timing charts showing a crank angle position detection signal (to be abbreviated as SGT hereinafter) which is the output signal of the crank angle position detection sensor 110 and a cam angle position detection signal (to be abbreviated as SGC hereinafter) which is the output signal of the cam angle position detection sensor 111. FIG. 9 shows the phase relationship between SGT and SGC and how to calculate an actual valve timing detection value .theta.a. SGC* of FIG. 9(b) is SGC when the valve timing is at the position of the slowest angle and SGC* of FIG. 9(c) is SGC when the valve timing is at the position of the advance angle.
The ECU 122 measures a time T.sub.110 corresponding to 110.degree. CA (the displacement angle of the crank shaft when the crank shaft turns once per 1 rotation of the cam shaft) in SGT of FIG. 9(a) and a phase difference time Ta between SGC and SGT to obtain an actual valve timing detection value .theta.a from the following equation (1) at each SGT timing (for example, every BTDC (before top dead center) 75.degree. CA). EQU .theta.a=Ta/T.sub.110.times.110[.degree. CA] (1)
Under the condition of a stable predetermined operation state such as idling, the actual valve timing detection value .theta.a when the target advance angle is the slowest angle position (.theta.b=0) is stored as a first storage value .theta.*1 (slowest angle learned value). The slowest angle learned value .theta.*1 serves as a reference value for the calculation of the actual advance angle .theta.r of the actual valve timing, is set to absorb a detection difference for each system caused by differences in parts such as the VVT ACT 114, the crank angle position detection sensor 110 and the cam angle position detection sensor 111 and differences in attachment and updated frequently at very short intervals, for example, 25 ms or SGT timing (for example, BTDC 75.degree. CA) for high-accuracy control.
In fact, the slowest angle learned value .theta.*1 is limited to a range between upper and lower limits in consideration of differences in parts such as the VVT ACT 114, the crank angle position detection sensor 110 and the cam angle position detection sensor 111 and differences in attachment (for example, upper limit value .theta.*max=45.degree. CA and lower limit value .theta.*min=5.degree. CA when design center value.+-.difference=25.+-.20.degree.).
That is, the actual advance angle .theta.r which is the advance angle of the actual valve timing based on the slowest angle learned value .theta.*1 can be obtained from the following equation (2). EQU .theta.r=.theta.a-.theta.*1 (2)
The ECU 122 carries out feed-back control by means of the control means 6 based on a difference between the actual advance angle .theta.r and the target advance angle .theta.b to converge the actual advance angle .theta.r to the target advance angle .theta.b.
Further, the ECU 122 detects a fail of the valve timing changing unit 7 which consists of the VVT ACT 114 and the OCV 121 by the following method.
On the contrary, when such a fail (abnormality in the slowest angle) occurs that the position of the spool valve of the OCT 121 is fixed and the VVT ACT 114 sticks to the valve timing of the slowest angle position of the intake valve, the actual advance angle .theta.r=.theta.a-.theta.*1=0.
The fail judging means 8 judges and detects that a fail occurs when the difference between the target advance angle .theta.b and the actual advance angle .theta.r, which is larger than a fail decision advance angle .theta.f1 (for example, 20.degree. CA), that is, .vertline..theta.b-.theta.r.vertline.&gt;.theta.f1 is continued for a fail decision time tf1 (for example, 5 sec) or more.
That is, although feed-back control is carried out to converge .theta.r to .theta.b, when .vertline..theta.b-.theta.r.vertline.&gt;.theta.f1 is continued after the passage of the fail decision time tf1, it is judged that abnormality occurs and a fail is detected.
In a technology disclosed by Japanese Laid-open Patent Application No. 8-200020, since the value of actual displacement angle VTB equivalent to the actual valve timing detection value .theta.a is updated as the slowest angle learned value GTV equivalent to the storage value of the first storage means by learning (step S15 of FIG. 4 of Japanese Laid-open Patent Application No. 8-200020), the slowest angle learned value GTV is sequentially updated and erroneously learned. Therefore, the displacement angle VT=VT-GTV (that is, .theta.r=.theta.a-.theta.*1) equivalent to the above actual advance angle .theta.r becomes smaller than the actual advance angle and .theta.r.ltoreq..theta.f1, thereby making it impossible to judge a fail.